p143
Astragalus

Article by Philip Smith, B.A., of the University of London
on pp143‑144 of

ASTRA′GALUS
(ἀστραγαλος), literally signifies that particular bone in the ankles of certain quadrupeds, which the Greeks, as well as the Romans, used for dice and other purposes, as described under the corresponding Latin word
Talus.

As a Latin word, astragalus is used by Vitruvius, who of course borrowed it from the Greek writers on architecture, for a certain moulding (the astragal) which seems to have derived its name from its resemblance to a string or chain of tali; and it is in fact always used in positions where it seems intended to bind together the parts to which it is applied. It belongs properly to the more highly decorated forms of the Ionic order, in which it appears as a lower edging to the larger mouldings, especially the echinus (ovolo), particularly in the capital, as shown in the following woodcut, which represents an Ionic capital found in the ruins of the temple of Dionysus at Teos. Still finer examples occur in the capitals of the temples of Erechtheus and Athene Polias, at Athens, where it is seen, too, on the sides of the volutes. It is also often used in the entablature as an edging to the divisions of the cornice, frieze, and architrave. The lower figure in the woodcut represents a portion of the astragal which runs beneath the crowning moulding of the architrave of the temple of Erechtheus. It is taken from a fragment in the British Museum, and is drawn of the same size as the original.

The term is also applied to a plain convex moulding of the same sectional outline as the former, but without the division into links, just like a torus on a small scale: in this form it is used
p144 in the Ionic base [Spira]. In the orders subsequent to the Ionic, — the Corinthian, Roman Doric, and Composite, — the astragal was very freely used. The rules for the use of the moulding are given by Vitruvius (III.5 §3,
IV.6 §§2, 3
Schneid.). Numerous fine examples of it will be found in the plates of Mauch (Die Griechischen und Römischen Bau-Ordnungen, Potsdam, 1845).

(In the printed edition, the lower band is shown 51 mm long.)

Images with borders lead to more information.
The thicker the border, the more information.
(Details here.)